1. Field of the Invention
The present invention relates to methods for processing semiconductor substrates, and particularly to a method for processing semiconductor substrates in which contamination by impurities is prevented.
2. Description of the Background Art
Semiconductor substrates (e.g., silicon substrates) are cleaned as pretreatment prior to semiconductor device manufacturing process. The RCA cleaning process is commonly used for the cleaning. In the RCA cleaning process, semiconductor substrates are subjected to an SPM cleaning process using a mixture of sulfuric acid (H.sub.2 SO.sub.4), hydrogen peroxide solution (H.sub.2 O.sub.2), and pure water (H.sub.2 O) as cleaning solution, an APM cleaning process using a mixture of ammonia (NH.sub.4 OH), hydrogen peroxide solution (H.sub.2 O.sub.2), and pure water (H.sub.2 O) as cleaning solution, and an HPM cleaning process using a mixture of hydrochloric acid (HCl), hydrogen peroxide solution (H.sub.2 O.sub.2), and pure water (H.sub.2 O) as cleaning solution.
The SPM cleaning is suitable for removal of organic matter, the APM cleaning is suitable for removal of organic matter and heavy metals, and the HPM cleaning is suitable for removal of heavy metals. Cleaning process with pure water is performed between the individual cleaning processes described above. Only the SPM cleaning process and the APM cleaning process may be applied instead of applying all of the three kinds of cleaning processes
While the RCA cleaning process removes organic matter and heavy metals away from the surface of a semiconductor substrate, an oxide film is formed on the semiconductor substrate surface during the cleaning process. The film is formed of native oxide naturally formed by dissolved oxygen in pure water and oxidizing action of hydrogen peroxide solution. This phenomenon is described in Japanese Patent Laying-Open No. 7-86220, Japanese Patent Laying-Open No. 5-29292, and Japanese Patent Laying-Open No. 63-29516.
FIG. 7 and FIG. 8 show results obtained by analyzing a native oxide film on a semiconductor substrate by SIMS (Secondary Ion Mass Spectrometry). FIG. 7 shows the results obtained by analyzing boron in a native oxide film on a semiconductor substrate immediately after application of the RCA cleaning. FIG. 8 shows the results obtained by analyzing boron in a native oxide film on a semiconductor substrate left in clean air for several hours after application of the RCA cleaning process. In either of the drawings, the abscissa shows depth (.mu.m) and the ordinate shows concentration (Atom/cm.sup.3).
As shown in FIG. 7, immediately after the RCA cleaning process, the concentration of boron in the oxide film is not higher than the lowest limit of detection and is hidden in the background. However, as shown in the area X in FIG. 8, the concentration of boron becomes higher in the vicinity of the surface when it is left in clean air. This means that boron contained in air has been taken into the native oxide film on the semiconductor substrate. Needless to say, if boron acting as a semiconductor impurity moves into semiconductor layers after formation of semiconductor devices, it will exert influence on the characteristics of the semiconductor devices. Especially, when semiconductor substrates are moved into semiconductor device manufacturing process without removing the native oxide film after cleaning, it is necessary to take the utmost care to avoid inclusion of boron in the oxide film. However, as stated above, there is the problem that the native oxide film formed during the cleaning process of semiconductor substrates takes in boron when left in air. For other substances existing in air and affecting semiconductor characteristics, it is also necessary to avoid inclusion of phosphorus and sodium in the oxide film.